Control valves are commonly used to control the fluid passing through a pipe. As known to those skilled in the art, a control valve regulates the rate of fluid flow as the position of the valve plug is changed by an actuator. There are three important features of a control valve, such as a globe valve, that it must contain the fluid without external leakage, that it must be able to withstand the pressure and temperature of the fluid, and that the fluid forces acting on the plug or closure element should not cause instability nor prevent the plug from shutting the valve off.
Quite often control valves have to perform under elevated pressure and temperature conditions. While valves with a single seat shut-off configuration and an unbalanced plug provide good service in these applications, they do require substantial forces that have to be provided by an actuator in order to overcome the effects of high fluid pressure acting on such unbalanced plug surfaces. For example, a one inch valve plug needs more than 800 lbs force to close against 1000 psi fluid pressure.
Such actuator force requirements can be quite expensive and such valves face other problems such as stability of operation. A typical example of such a single seated valve is shown in U.S. Pat. No. 4,474,356.
One typical way of alleviating these problems in prior art is the use of a cage with dual seating surfaces for the plug. My prior inventions, shown in U.S. Pat. Nos. 6,536,472 B2 and 3,805,839, describe such a solution. While capable of performing the required tasks, this is a very expensive solution due to the extra, precision machined, parts and due to the extra heavy bonnet bolting that is required to lock the cage within the valve housing. Finally, these valves have an additional problem of absorbing thermal expansion of the cage, especially if cage and housing have to be made of different materials. Another added expense is posed by the requirements to have the seating surfaces of the plugs lapped against opposing surfaces in the seat rings in order to obtain tight shut-off.